Bimetallic M/N/C catalysts prepared from π-expanded metal salen precursors toward an efficient oxygen reduction reaction

Nonprecious metal electrocatalysts are being explored as alternatives to platinum-group metal electrocatalysts for the oxygen reduction reaction (ORR) which is required for cathode materials in fuel cells. Herein, we describe a new method for preparing bimetallic nitrogen-containing carbon catalysts with high ORR activity using π-expanded M(salen) precursors. The M/N/C and bimetallic FeM/N/C ORR catalysts were obtained by pyrolysis of a mixture of a carbon support (Vulcan XC-72R) and the metal complex as a precursor. The bimetallic FeCu catalyst prepared from Fe and Cu complexes with the N,N′-bis(2-hydroxy-1-naphthylidene)-1,2-phenylenediamine ligand (2NAPD) is found to have an onset potential of 0.87 V, which is positively shifted by 50 mV from that of the catalyst prepared from the monometallic Fe(2NAPD) complex. The FeCu/N/C catalyst promotes efficient four-electron reduction in the ORR. High-resolution transmission electron microscopy studies reveal that both Fe and Cu metals together with pyridinic nitrogen species are highly dispersed within the carbonaceous structure in FeCu/2NAPD@VC, suggesting that the N-coordinated Fe and Cu sites promote efficient four-electron reduction of O2. This new methodology facilitates design of nonprecious bimetallic carbon catalysts with excellent ORR activity.


Introduction
Electrochemical reduction of O 2 is a pivotal reaction for improving the performance of energy conversion devices such as fuel cells, metal-air batteries, and electrolyzers. [1][2][3] In particular, polymer electrolyte fuel cells (PEFCs) have been recognized as efficient energy converters enabling low emissions and low environmental impact. Precious metal group (PMG) catalysts have been the most widely used catalysts for the cathodic oxygen reduction reaction (ORR) in PEFCs. [4][5][6] However, the high cost and low abundance of precious metals have been the main obstacles facing widespread commercialization of PEFCs. Thus, extensive investigations have been reported for development of alternative low-cost non-precious metal catalysts (NPMCs). [7][8][9] Catalysts including a rst row transition metal, such as Fe, Cu, or Co, and nitrogen atoms embedded within a carbonaceous structure are known as M/N/C catalysts and have been generally considered to represent promising alternatives to PGM catalysts, because the M/N/C catalysts have been shown to promote high levels of ORR activity while having suitable durability for use in PEFCs. [10][11][12][13][14][15] Pyrolyzed transition metals such as Fe, Cu, and Co in combination with macrocyclic ligands adsorbed on carbon supports have been proven to improve ORR activity and stability under acidic conditions, which is a requirement for PEFCs. 16,17 A series of low-cost nitrogencontaining precursors such as phenanthroline, 11 polypyrrole, 10 and polyaniline 12 have been utilized in preparation of M/N/C catalysts. The exact chemical structures of the metalcontaining active sites in the M/N/C catalysts prepared by pyrolysis remain under debate, 18,19 although a metal and nitrogen-containing active site structure has been proposed as a catalytic site on the basis of data obtained from various spectroscopic techniques. [20][21][22][23] It is believed that metals coordinated by the N-groups play important roles in enhancement of the ORR activity of M/N/C catalysts. 11,24 Inspired by cytochrome c oxidase, which catalyzes O 2 reduction using a unique bimetallic Fe and Cu active site with ultimate efficiency, a number of examples of construction of bimetallic M/N/C catalysts containing rst row transition metals as active sites have been reported. [25][26][27][28][29][30][31][32] In the enzyme, the active site containing heme iron and a copper ion is known to efficiently promote four-electron reduction of a bound O 2 molecule to water. Thus, M/N/C catalysts prepared from two precursors with two metal ions have been investigated in attempts to improve the ORR activity. [33][34][35] Previously, we reported preparation of Fe/N/C catalysts having high ORR activity by systematically tuning an aromatic framework of Fe(salen) complex precursors. 36 We demonstrated that the designed Fe(salen) derivatives with various aromatic moieties such as 1NAPD and 2NAPD ligands have higher thermal stability, which affects the annulation process of the complexes during pyrolysis, thereby leading to higher ORR activity. In this work, we describe preparation and characterization of new bimetallic FeM/N/C catalysts via pyrolysis of the p-expanded metal salen complexes M(2NAPD) and measurement of their ORR activity. It is found that combining the M(2NAPD) precursors, in particular Fe and Cu, leads to a positive shi of onset potentials of the ORR activity of the pyrolyzed bimetallic M/N/C catalysts.
The M/N/C electrocatalysts were prepared by pyrolysis of the mixture of M(2NAPD) complexes and carbon black Vulcan XC-72R (VC, Cabot, USA) (Fig. 1b). One or two types of the M(2NAPD) complex dissolved in CHCl 3 were vigorously mixed with VC and the solvent was removed to afford the precursor. The obtained precursor was preheated to 300 C for 1 h and then incubated at 300 C for 2 h under constant N 2 gas ow. The carbon materials were immediately pyrolyzed at 1000 C for 2 h under constant N 2 gas ow for carbonization. The pyrolyzed powder was ground and leached in an acidic solution (0.5 M H 2 SO 4 ) to remove metal species such as metal oxides and unincorporated metal species. Aer washing twice with an excess of water and drying, Fe/2NAPD@VC, Cu/2NAPD@VC, Co/ 2NAPD@VC, Ni/2NAPD@VC, Mn/2NAPD@VC, FeCu/ 2NAPD@VC, FeCo/2NAPD@VC, FeNi/2NAPD@VC, and FeMn/ 2NAPD@VC were produced. These materials were characterized by elemental analysis, inductively coupled plasma atomic emission spectrometry (ICP-AES), X-ray diffraction (XRD), and Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).

ORR activity
The ORR activities of the M/N/C and bimetallic FeM/N/C catalysts were determined using a rotating disk electrode at different rotation rates in a medium of O 2 -saturated 0.1 M First, in the case of the monometallic M/N/C catalyst, as we reported previously, the onset potentials of the Fe/N/C catalysts shi positively for the catalyst prepared from the Fe(salen) complex with the p-expanded ligand framework. In particular, Fe/2NAPD@VC has a signicantly shied onset potential, suggesting that introduction of aromatic rings into the ligand framework of the precursor positively shis the onset potential in Fe/2NAPD@VC relative to Fe/salen@VC. 36 Among the M/N/C catalysts prepared in this work, Fe/2NAPD@VC has the most shied positive onset potentials (0.82 V) relative to the onset potentials of other catalysts including Cu/2NAPD@VC, Co/ 2NAPD@VC, Ni/2NAPD@VC, and Mn/2NAPD@VC (Table 1). The average number of electrons transferred during O 2 reduction is 3.6 for Fe/2NAPD@VC, generally indicating occurrence of four electron reduction. Both Cu/2NAPD@VC and Co/ 2NAPD@VC also show moderate activity for four electron reduction. In contrast, Ni/2NAPD@VC and Mn/2NAPD@VC promote two electron reduction as well as metal-free 2NAPD@VC.
On the basis of the electrochemical result showing high electrocatalytic ORR activity for Fe/2NAPD@VC, we prepared bimetallic FeM/N/C catalysts using Fe(2NAPD) and other M(2NAPD) complexes as precursors and their electrocatalytic properties were analyzed. The onset potentials of FeCu/ 2NAPD@VC, FeCo/2NAPD@VC, FeNi/2NAPD@VC, and FeMn/ 2NAPD@VC are 0.87 V, 0.83 V, 0.76 V, and 0.69 V, respectively. Interestingly, the onset potential (0.87 V) of FeCu/ 2NAPD@VC exhibits remarkable positive shis relative to the onset potential of Fe/2NAPD@VC (0.82 V). In addition, FeCo/ 2NAPD@VC also has a slightly shied onset potential (0.83 V). Furthermore, the n value of FeCu/2NAPD@VC (3.9) is higher than that of Fe/2NAPD@VC (3.6). The results clearly indicate that the bimetallic catalyst FeCu/2NAPD@VC promotes four electron reduction of O 2 and has higher activity relative to that of monometallic Fe/2NAPD@VC and Cu/2NAPD@VC. We also found that the 1 : 1 ratio of the Fe and Cu 2NAPD complexes has the best activity ( Fig. S4a †). Furthermore, the bimetallic Fe/Cu carbon catalyst prepared from the mixture of simple Fe and Cu salen complexes has a negatively shied onset potential of 0.77 V (Fig. S4b †). These ndings indicate that the ORR catalysts prepared from the mixed M(2NAPD) precursors have a synergistic effect provided by bimetallic active sites, which efficiently proceed four-electron reduction. Since the onset potential for ORR shis negatively to 0.84 V aer 100 cycles, the durability of FeCu/2NAPD@VC was not excellent (Fig. S9 †).

Characterization of the M/N/C and FeM/N/C catalysts
The carbon materials in the monometallic M/N/C and bimetallic FeM/N/C catalysts were rst analyzed by Raman spectroscopy ( Fig. 3a and 4a). It is known that a disordered carbon structure has signicantly different spectra with a D (disorder) band in the vicinity of 1355 cm À1 . Graphitized carbons give rise to a G (graphite) band in the vicinity of 1580 cm À1 , which is assignable to in-plane displacement of carbon strongly coupled in the hexagonal sheets. 41,42 The Raman spectra of the M/N/C and bimetallic FeM/N/C catalysts presenting typical D-and Gbands indicate that the disordered carbonaceous structure is included in these catalysts. The integrated intensity ratio I D /I G for the D-band and G-band is widely used to quantify defects in graphitic materials. The I D /I G ratio for all monometallic M/N/C and bimetallic FeM/N/C catalysts is approximately 1.1, suggesting that all catalysts include similar graphitic and Table 1 Characterization and electrochemical activity of M/N/C catalysts   The nitrogen-doped graphene has been known to have D 0 peak near 1600 cm À1 . [43][44][45] The G band peaks for the M/N/C catalysts are located at 1600 cm À1 , which represents a shi from the peak at 1583 cm À1 for VC without nitrogen atom and for VC pyrolyzed with the 2NAPD ligand. This result indicates that nitrogen atoms are doped in the carbon framework in accordance with the metal species. X-ray diffraction (XRD) experiments were performed to conrm each carbonaceous structure of the M/N/C and FeM/N/ C catalysts prepared from the M(2NAPD) complexes and the chemical composition of the metal species (Fig. 3b and 4b). In general, powder samples of carbons in an amorphous structure provide diffraction peaks for (002) (2q ¼ ca.  Tables 1 and 2 together with other properties. The content of the metal ranges between 1.2-0.1%. Therefore, the metals of the precursors are retained in the carbonaceous structure aer treatment of an aqueous H 2 SO 4 solution at 80 C to remove the metal nanoparticles and metal oxides. In addition, the nitrogen content of the M/N/C catalysts is higher than that of the 2NAPD@VC catalyst prepared from 2NAPD without metal (0.3%). This suggests that the larger amount of the nitrogen species is incorporated into the carbonaceous structure by the pyrolysis of the metal salen precursors.
High resolution transmission electron microscope (HRTEM) images of the M/N/C and FeM/N/C catalysts were obtained to   Paper determine the nanostructures of the catalyst (Fig. 5 and 6). The M/N/C catalysts are found to have a spherical form with diameters ranging from 40 to 70 nm, which are retained in the structure of VC. We conrmed the existence of each metal in the carbon structure by performing chemical composition analyses using EDS (Energy dispersive X-ray spectrometry). The data are in good accordance with the data obtained from the ICP-AES measurements. Therefore, the main components of metals are highly dispersed in the carbon structure in each catalyst. In the case of Co/2NAPD@VC, FeCo/2NAPD@VC, and FeNi/ 2NAPD@VC, we also observed nanoparticles of Co metal, FeCo, and FeNi alloys, respectively in HRTEM images (Fig. S6 †).
In contrast, such nanoparticle of FeCu alloys were not observed at all in the FeCu/N/C catalyst (Fig. S7 †), which shows a similar trend observed in Fe/2NAPD@VC and Cu/2NAPD@VC. Considering the existence of the FeCu alloy structure in XRD analysis, the results indicate that highly dispersed FeCu alloys are generated in FeCu/2NAPD@VC, which has the highest activity in four electron reduction of O 2 .

XPS measurements
The chemical structures of the nitrogen atoms in the M/N/C and FeM/N/C catalysts were determined by X-ray photoelectron spectroscopy (XPS) measurements ( Fig. 7 and 8).  Tables 1 and 2. Interestingly, the content of pyridinic N of Fe/2NAPD@VC (24.9%) and Cu/2NAPD@VC (21.2%) is higher than that of the catalyst prepared from 2NAPD (8.9%). This indicates that the pyridinic moiety in the carbonaceous structure is important to bind the metal active sites in the catalysts. This is consistent with the reported evidence that metals coordinated by the N-groups play important roles in enhancement of the ORR activity of M/N/C catalysts. 11,24 We found that the nitrogen content (0.8 wt%) and the pyridinic nitrogen content (20%) for FeCu/2NAPD@VC are higher than those of other catalysts. Since the catalysts are treated in acid during the preparation, only the carbonencapsulated metal or metal alloy nanoparticles remain in the carbonaceous structure. Therefore, we expect that the N-

Conclusions
This work demonstrates a new method for improving the ORR activity of bimetallic M/N/C catalysts using mixed metal precursors containing p-expanded ligand frameworks. The bimetallic FeM/N/C catalysts were prepared by pyrolysis of the mixture of carbon support and the M(2NAPD) precursors. The catalyst prepared using the Fe(2NAPD) complex with pexpanded ligand framework exhibits a remarkable positive shi in the onset potential for ORR relative to catalysts prepared from the M(2NAPD) complex (M ¼ Cu, Co, Ni, Mn). By mixing these heterometallic precursors, we found that ORR activity can be signicantly improved. In particular, FeCu/2NAPD@VC has a positively-shied onset potential for ORR with excellent fourelectron O 2 reduction activity. Both Fe and Cu metals together with pyridinic nitrogen species are highly dispersed within the carbonaceous structure in FeCu/2NAPD@VC, suggesting that the N-coordinated Fe and Cu sites promote efficient fourelectron reduction of O 2 . These ndings prove that our approach of using aromatic ligand frameworks for metal precursor complexes contributes to enhancement of ORR activity of non-precious bimetallic M/N/C catalysts which are appropriate for incorporation into fuel cells and metal-air battery applications.

Experimental section
Catalyst preparation

Physicochemical characterization
The decomposition temperatures of M(2NAPD) complexes were determined by thermogravimetry-differential thermal analysis (TG-DTA) using a Mac Science TG-DTA TMA DSC with a heating rate of 10 C min À1 under an N 2 stream in a platinum pan. Metal content of each of the M/N/C catalysts and the FeM/N/C catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) using a SHIMAZDU ICPS-7510 system. Specic surface areas were obtained using a Quantachrome NOVA 4200e Surface Analyzer and calculated by the Brunauer-Emmett-Teller (BET) method. The Raman spectra were obtained using a JASCO NRS-3100 instrument with a 532 nm laser. X-ray diffraction (XRD) patterns of the samples were obtained using an X-ray diffractometer (Rigaku, SmartLab) equipped with a Cu Ka source. High-resolution transmission electron microscopy (HRTEM) observations and chemical composition analyses were carried out using a HITACHI HF-2000 eld emission TEM operated with an accelerating voltage of 200 kV. The chemical composition of each sample was analyzed by EDS (NORAN Instruments). The analyses were carried out using an electron probe approximately 25 nm in diameter. The characteristic X-ray of metals was collected with an ultra-thin window X-ray detector at a high take-off angle of 68 degrees. The morphology of the catalysts were observed using a JEOL JSM-6335 eld emission scanning electron microscope (FE-SEM) operated at an accelerating voltage of 20 kV. XPS measurements were performed on a KRATOS AXIS-165x (SHI-MADZU) system, equipped with a Mg Ka X-ray source. Individual chemical components of the N 1s binding energy region were tted to the spectra aer a Tougaard-type background subtraction.

Electrochemical measurement
A rotating ring-disk electrode with a glassy carbon disk electrode (f ¼ 5 mm) and platinum ring was used for evaluation of the carbon catalysts. Electrode rotation rates were controlled using a Pine Instruments AFMSRCE rotator with a Pine MSRX motor controller. The electrode was polished to mirror at with alumina powder (50 nm) before use. The catalyst ink included a mixture of 12.0 mg of catalyst ((Fe/2NAPD@VC, Cu/ 2NAPD@VC, Co/2NAPD@VC, Ni/2NAPD@VC, Mn/ 2NAPD@VC, 2NAPD@VC, FeCu/2NAPD@VC, FeCo/ 2NAPD@VC, FeNi/2NAPD@VC, or FeMn/2NAPD@VC)), and 50 mL of 5 wt% Naon® solution (Sigma-Aldrich), and 950 mL of isopropanol. The ink was vortexed and sonicated in an ultrasonic bath at 100 W at 35 kHz for 30 min. Then 10 mL of the catalyst ink was loaded onto the surface of the electrode and dried.
Electrochemical tests were carried out on a potentiostat (ALS, electrochemical analyzer model 610B) using a typical three-electrode system, with platinum wire as an counter electrode and Ag/AgCl as a reference electrode. The potential difference between Ag/AgCl and RHE was calculated and the value is 0.258 V in a 0.1 M HClO 4 solution. The scan rate for all measurements was 5 mV s À1 from À0.258 to 0.742 V versus the Ag/AgCl reference electrode. Before each potential scan, the electrolyte of the 0.1 M HClO 4 solution was saturated with O 2 for at least 30 min, and O 2 purging was continued during the electrochemical experiments. The measured current was subtracted from the background current at the N 2 -saturated electrolyte. The number of electrons transferred during O 2 reduction was calculated using the Koutecky-Levich equation (eqn (1) and (2)) 54 I À1 ¼ I K À1 + I L

Conflicts of interest
There are no conicts to declare.